Some setups with an independent USB to a baseband module can still control the bus to other devices by exploiting USB host modes. (i.e. if the log output indicates it can no longer control/communicate with the usb, or the usb disconnected, then it is very likely the usb bus was hijacked by another device).
Others phones still have some type of bus overlap, and they provide no way to block the baseband chip from accessing other devices on the shared bus.
Even then, this is not an easy task as the baseband chip would be competing with the processor chip in accessing devices.
edit: It is possible in future revisions, SoC Chips in mobile phones in the US will include capabilities to control the phone irrespective of the running soft OS. As of now there are very few phone manufacturers that include open access to the phone's hardware, and the processor. It's a hunch the US companies will try to blockade open phones, as foreign phones, particularly Samsung/LG/HTC, are providing a way for secure phones to be developed. Apple's attempts and level of aggression to blockade Samsung all around the world was incredibly suspicious, to say the least, and only S.Job sycophants tend to think it was for 'competitive' value. The US is fast becoming a strange place.
Back in the days there was a HF hack that allowed a remote operator to listen in on conversations in the room by using the fact that such signals would totally ignore the 'on hook' disconnect of old style 'wired' phones ('POTS'), these baseband processors are a much worse instance of the same phenomenon. Not that it matters much, after all, if you have access to the cell phone towers you can listen in on whatever you want, making pictures is of a lower threat level (to me) than listening in on random conversations in the vicinity of the phone.